The ability of complex organisms to sense gravity, motion, and spatial orientation is a crucial function enabled by the vestibular system. The proposed studies will investigate the molecular basis of a mouse mutation head tilt (het), which has disrupted gravity receptor function. In preliminary studies, the pathology of this mutant has revealed a complete absence of otoconia, the calcium carbonate crystals that stimulate the vestibular maculae during linear acceleration and gravity. The goals of this application are straightforward and focused. We intend to analyze the het mutant phenotype and to clone the het gene in order to understand its function in otolith development. The proposed studies aim to: 1) complete a physical map of the het-critical region and identify and characterize the het gene; 2) further characterize the het mutant phenotype by histological, electron microscopic, and embryonic stem (ES) cell approaches; and 3) analyze otoconial maintenance using conditional gene expression technologies. Together these experiments will provide a clearer understanding of vestibular function and disease.